The adhesion of coatings applied directly to the surface of a substrate metal is of special concern when the coated metal will be utilized in a rigorous industrial environment. Careful attention is usually paid to surface treatment and pre-treatment operation prior to coating. Achievement particularly of a clean surface is a priority sought in such treatment or pre-treatment operation. Representative of a coating applied directly to a base metal is an electrocatalytic coating, often containing a precious metal from the platinum metal group, and applied directly onto a metal such as a valve metal. Within this technical area of electrocatalytic coatings applied to a base metal, the metal may be simply cleaned to give a very smooth surface as taught in U.S. Pat. No. 4,797,182. Treatment with fluorine compounds may produce a smooth surface as taught in U.S. Pat. No. 3,864,163. Cleaning might include chemical degreasing, electrolytic degreasing or treatment with an oxidizing acid as taught in U.S. Pat. No. 3,864,163.
Cleaning can be followed by mechanical roughening to prepare a surface for coating as taught in U.S. Pat. No. 3,778,307. If the mechanical treatment is sandblasting, such may be followed by etching as taught in U.S. Pat. No. 3,878,083 or pickling with a non-oxidizing acid can produce a rough surface for coating as taught in U.S. Pat. No. 3,864,163. Such pickling can follow degreasing as taught in U.S. Pat. No. Re. 28,820. The pickling may readily etch titanium to a surface roughness within the range of 150-200 or more microinches. "Titanium as a Substrate for Electrodes", Hayfield, P. C. S., IMI Research and Development Report.
If there is a pre-existing coating present on the substrate metal, the metal can be treated for coating removal. For an electrocatalytic coating, such treatment may be with a melt containing a basic material used in the presence of an oxidant or oxygen. Such can be followed by pickling to reconstitute the original surface for coating as taught in U.S. Pat. No. 3,573,100. Or if a molten alkali metal hydroxide bath is used containing an alkali metal hydride, this is preferably followed by a hot mineral acid treatment as taught in U.S. Pat. No. 3,706,600. It has also been proposed to prepare the surface without stripping the old coating as taught in U.S. Pat. No. 3,684,543. More recently, this procedure has been improved by activation of the old coating, prior to application of the new as taught in U.S. Pat. No. 4,446,245.
Another procedure for anchoring the fresh coating to the substrate, that has found utility in the application of an electrocatalytic coating to a valve metal, is to provide a porous oxide layer which can be formed on the base metal.
It has, however, been found difficult to provide long-lived coated metal articles for serving in the most rugged commercial environments, e.g., oxygen evolving anodes for use in the present-day commercial applications utilized in electrogalvanizing, electrotinning, copper foil plating, aluminum anodizing, sodium sulfate electrolysis, electroforming or electrowinning. Such may be continuous operation. They can involve severe conditions including potential surface damage. It would be most desirable to provide coated metal substrates to serve as electrodes in such operations, exhibiting extended stable operation while preserving excellent coating adhesion. It would also be highly desirable to provide such an electrode not only from fresh metal but also from recoated metal.